In Lightroom Classic, you can only sync a simple collection with Lightroom CC. If you want to sync all your photos with Lightroom CC, you need to create a simple collection that contains all your photos. This is simple the first time around (create the collection and drag all your photos into it). Maintaining it is difficult, though.
These instructions can help:
- In the Catalog panel on the left, click All Synced Photographs.
- Do Edit > Select All.
- In the Catalog panel, click All Photographs.
- Do Photo > Stacking > Expand All Stacks.
- Do Edit > Invert Selection. The unsynced photos are now selected.
- Drag the selected photos to your “All Photos to Sync” collection.